Measuring instrument



H. R; EGGE RS MEASURING INSTRUMENT Filed Nov. 8, 1935 A R m mm Maw was ea a 2; if. DADD 8v 5w Q mi $3 tPtt l\l\ H234 Aug. Y 31, 1937..

I Inventor: Hermann R. Egger's; by W 5 fiwl His. Attorney.

Patented Aug. 31, 1937 s 2,091,848 MEASURING INSTRUMENT Hermann R. Egger's, Berlin, Germany, assilnor to General. Electric Company, a corporation oi New York I Application November 8, 1935, Serial No. 4:,912 In Germany December 18, 1934 9 Claims. (Cl. rs-az) -Myinvention relates to measuring instruments the claims appended hereto. In the drawing and concerns particularly devices responsive to Fig. 1 is a schematic diagram of one form of my the degree of superheat of steam and other vapors invention representing the electric circuit, and or gases. 1 a Fig. 2 is a set of graphs explaining the theory When saturated steam is conveyed in pipes for of operation of the apparatus. 6 any considerable distance, the steam is liable to Fig. 1 illustrates an arrangement for measuring condense in the pi es. In order to avoid conthe superheat of steam at a point II in a pipe densation when sending steam through long through which steam issues from a boiler l2 lengths of piping and also for other purposes. having uperheatin @0115 There is Wheat- .1 UNITED STATESPATENT OFFICE steam'is frequently superheated. When the steam stone b e m e up O a p a y of resistors- 1 pressure is constant, the temperature of the Two of the bridge arms are formed by resistors steam indicates the degree of superheat. This is l and 5, having substant a ly c nstant esistnot the case, however, when the steam pressure i e A third bridge a m nc udes a es stor varies. If the pressure increases while the temwh s c se t 3 in resistance n ac o dperature. remains the same, the superheating beance with variations in steam temperature. For 15 comes less, and vice versa. It is, therefore, diiile p e, t e ird m y be rmed by a recult in variable pressure installations to obtain sistor l6 which is kept at the temperature oi the a conception of the degree of superheat simply team- The r sistor s is kep at h t mpera from thermometer readings, ture of the steam in any suitable manner as by In accordance with my invention, I provide both Providing a j ck w 8 steam Passage in illti- 90 temperature and pressure responsive elements and mate t mal contact W t resistor i 0 y caus their action to be combined with a steam placing the resistor is direc ly within the p ne chart in such a way that the degree of superheat A fourth bridge arm is made p 0! several may readily be perceived. The steam chart may I r ist rs. r pr s nt d l e tiv y y the sym l 1'- either be actually drawn out as a suitable system The bridge arm r consists of a p i q of 25 of curves appearing under a pair of pointers, retresistor in Da e wi h the sponsive respectively to temperature and presportion q of the reheostat l1, and a resistor i9 sure, so as to permit directly readingthe superin series with the parallel pair of resistances q heat, or parts of the indicating apparatus may be and ID. The reheos'tat I1 is provided with a slidarranged to act in accordance with the relationing contact 20, which is connected to one end of 50 less of pressure variations.

ships expressed by a steamchart. the resistor l8 and serves as one terminal of the In carrying out my invention in its preferred bridge. The contact 20 is arranged to slide over m, the relationships of the steam chart are the rheostat l! in accordance with variation in produced by the properties of electric circuit elepressure of the steam at H. For example, a-

bined electrically to provide a direct indication tube or an expanding diaphragm or a bellows 2|, of superheat by a single movable member, such is provided with a suitable gearing, such as a rack as a pointer or recording pen. A Wheatstone and pinion 22, for rotating the contact 20 in. bridge is provided with means for varying the reresponse to variations in steam pressure. sistance of one arm in response to variations in A source of current 23 is connected between the temperature of the steam, means for varying the opposite terminals 24 and 25 of the bridge toresistance in another arm in accordance with energize the bridge. Inadiagonal or cross circuit the pressure of the steam, and a bridge galva-' between the remaining two opposite terminals nometer responding to the relationship of the of the bridgeis' connected a current-conducting resistances in the variable resistance arms. It coil 26 of a galvanometer 21. The galvanometer will be evident that the galvanometer must read coil 26 isconnected to a bridge terminal 26 and zero when the steam is saturated regardless of a free end 29 of the reheostat I! so that the pressure, and that, at any given degree of supe galvanometer circuit includes the portion (rs-q) heat, the reading must-remain constant regardol' the rheostat ll, the symbol e being used to signify the entire resistance of the rheostat I1.

The invention will'be understood more readily It desired, the galvanometer 21 may be provided 1 from the following detailed description taken in with a voltage-compensating coil 26 connected I connection with the accompanying drawing, and across the current source 23 in order to compenthose features of the invention which are believed sate for variations in voltage of the source in the to be novel and patentable will be pointed out in manner of an ohmmeter, or ratio indicator.

ments, and pressure and temperature are compressure-responsive element, such as a Bourdon 35 5 For convenience, the graph is -75 T=F(ii), indicating that that the variable portion (e.-q) in the galvanom 1o eter circuit corrects for variations in bridge current resulting from the fact that the resistances of both arms i8 and r vary as the steam temperature and pressure vary for a given degree of superheat under variable steam pressure 15 conditions.

In Fig. 2, curve I, plotted from steam tables, represents a saturated steam pressure-temperature chart showing the variations in the pressure p; of saturated steam at various temperatures t 20 between 200 and 300 degrees centigrade, which is assumed, for the sake of illustration, to be the working range of theapparatus. temperatures, the pressure is found to vary from 15.85 atmospheres to 87.6 atmospheres absolute.

drawn with the axes of coordinates intersecting at the point representing 200 degrees centigrade and 15.85 atmospheres, so that the variables plotted are ac: tually (pr-15.85) and (t- -200). It will be observed that curve I is a sagging curve. The curve I may be represented symbolically by the equation p==f (t) indicating that 17s varies as a function of t.

For the sake of explaining the operation of the 35 apparatus, an example willbe given with certain numerical values assumed, although, of course, any desired circuit constants consistent with the principle of the invention may be employed. The

bridge arm i6 is assumed to be so designed that 40 its resistance in ohms, represented by the symbol 1', is given by the equation That is, the'resistanoe f varies linearly from 100* it is further assumed that the resistor l8 has a resistance of 84 ohms and the resistor I! has 55 a resistance of 100 ohms. Then,

'r==100+ill100+ -f L84- q 34 1.062(p-- 15.85) 6o 1 1 1.o62( -15.s5) This equation isplotted as curve 2 of Fig. 2.

The curve 2 of Fig.2 may be represented b the equation r=f(p),- indicating that the resistance of the bridge arm 1' varies as a function of the steam pressure. It will be seen that the curve 2 tends tobulge upward and the resistance 76 r varies from 1o0to'140 between the pressures 15.85 and:8'l.6. I

e If the values of 9 found from curve i are substituted in the equation of curve 2, r=f(p;), a new equation will be obtained, r'=f[f(t)l or r .is also some function Between these -to the fact that.

iore, tend to vary were which is very nearly true.

' of the temperature of saturated steam. The curve 3 representing the equation r'- -F(tr) may then be plotted. For example, following the dotted lines in Fig. 2 at the point 3i, when (t-200) =65, (p-15.85) =35.8. Carrying this value from the ordinate of curve I to the abscissa of curve 2 along the dotted line 32,9. value of (r--100) =26 is found from, curve 2. The point 13 may then be plotted at the intersection of the coordinates (t--200) =65 and (r--100) =26. The complete curve 3 may be'plotted in a similar way by locating additional points, and it will be found, owingto the fact that the curve I sags downward and the curve 2 bulges upward, that the curve 4 will be substantially linear with r varying from 100 ohms to 140 ohms between temperatures of 200 and 300 centigrade. However, I, the resistance of the bridge arm. It, also varies linearly through the same points, and a curve 4, J"=F(t) which might be drawn would coincide with curve 3, r=F(t,). I

- ,It is assumed also that resistances l4 and I5 are made equal although they may have any desired ratio if the, resistances of the other parts of the bridge are suitably chosen. Since curves 3 and l coincide, indicating that the bridge arms I and r have equal resistance at any pressure of saturated steam, the bridge will remain balanced and the galvanometer will correctly indicate zero degrees of superheat regardless of steam pressure when the steam is saturated.

When the steam is superheated, the temperature and resistanceof the raised although the steam pressure and the resistance of the arm 1 may remain unchanged. Consequently, current will flow in the galvanometer 21 and a reading of superheat will be given.

However, if the superheat remains constant as the pressure varies, the ratio between the resistances of arms 1" and 1- will tend to vary owing for any steam pressure, the resistance of the arm 1" remains a fixed amount higher than the resistance of the arm 1', or higher than the resistance which would have at the pressure in question if the steam were saturated. The reading of the galvanometer would, thereit not for the fact that the galvanometer current is augmented as the steam pressure rises by the decrease in resistance of the galvanometer circuit resulting from the fact that the resistance of the portion (e-q) of the rheostat i'l becomes smaller as the portion q becomes larger with increasing pressure. It will constant at a predetermined value of superheat regardless of the steam pressure;

If the galvanometer coil 26 is connected to the contact 20 instead of the terminal 29, the apparatus may still be used to indicate whether or not the steam is superheated although it will not give such accurate readings of the degree of superheat.

It has been assumed in the foregoing that the temperature coeflicient of resistance of the bridge arm 16 is a constant at different temperatures, However, if greater refinement is desired, any change with temperature in the temperature coeflicient of resistance may be taken into account by a suitable modification in the arrangement of the resistors.

In accordance with the provisions of the patent statutes, I have described the principle of operbridge arm I will be ation of my invention together with the apparatus which I now consider to represent the outby, other means.

What I claim as n'ew and desire to'secure by Letters Patent of the United States, is:

1. A steam-superheat responsive apparatus.

including a resistor of. temperature-responsive variable-resistance material 2. A steam-superheat indicator comprising in combination, a Wheatstone bridge including four resistance arms in series-parallel, means 'for varying the resistance of sponsei to variations in steam pressure, a current-responsive device connected to said Wheatstone bridge between the common point of one 40 pair of series bridge arms and the common point of the other pair of series bridgerarms, and means for energizing said bridge, fonepi said variable resistance arms having means for maintaining their resistance ratio constant under temr perature and pressure conditions corresponding sponsive to variation in the relative voltage drop distribution in said resistors, said resistors having a constant resistance ratio under temperature and pressure conditions corresponding to the temperature and pressure of saturated steam.

to steam pressure, a. source of current, a Wheatstone bridge including four impedance armsc'on nected vtosaid current source in series-parallel,

and a current-responsive device connected in cross circuit between said bridge arms, one of said bridge arms being variable in impedance and having impedance-adjusting means con-' trolled by said pressure-responsive device, another of said bridge arms being variable in impedance cross-circuit impedance g cuit impedance being increased as the pressure- 4. A steam-superheat responsive apparatus comprising in combination, a device responsive" p1 oportion and direction in response to steam temperature, and one '01 the latter two arms having means for maintaining their impedance ratio constant under temperature and. pressure conditions orresponding to the temperature and pressure ofsaturated team. V 5. A steam-superheat responsive apparatus comprising in combination, a source of current, a Wheatstone bridgeincludingi'our impedance arms connected to said current-source in series- I parallel, a current-responsive device and an im ing the impedance of another of said bridge arms and saidcross-circuit impedance in response to variations in steam pressure.

6. A steam-superheat responsive apparatus comprising in combination, a source of current, a

responsive impedance is decreased, and vice versa tially in proportion to variations in temperature of saturated steam with pressure.

8. A Wheatstone bridge sure respectively of steam and in such proportion bridge balanced while the steam,

as to keep; the

' is saturated.

-9. A Wheatstone bridge arrangement responsive to steam conditions" comprising in combination, a source of nected in series-parallel to said current source, a current-responsive device connected in cross circuit between said bridge arms so as to be responsive to unbalance thereof, and means for 'varying the impedances of two 'o f'said bridge "arms in response to variations in temperature of steam, and in such as to keep the bridge balanced while the steam is saturated. f

and pressure, respectively,

current, impedances con 

